@Path("idea")
public class IdeaController {

    @Context
    private HttpServletRequest request;
    
    @GET
    @Path("/test")
    @Produces(MediaType.APPLICATION_JSON)
    public String msg() {        
        HttpSession session = request.getSession();
        System.out.println(session.getId());
        return "{\"MSG\": \"Putin uhodi!!!\"}";
    }
}

MyNetworkService.getInstance().getJSONApi().test().enqueue(new Callback<Idea>() {
	@Override
	public void onResponse(Call<Idea> call, Response<Idea> response) {

	}

	@Override
	public void onFailure(Call<Idea> call, Throwable t) {
		onBackPressed();
		t.printStackTrace();
	}
});

public class Idea {
    @SerializedName("MSG")
    @Expose
    private String msg;
	
	 public String getMsg() {
        return msg;
    }

    public void setMsg(String msg) {
        this.msg = msg;
    }
}

public class MyNetworkService {

    private static MyNetworkService mInstance;
    private static final String BASE_URL = "http://localhost:8080/REST/";
    private Retrofit mRetrofit;

    private MyNetworkService() {
        OkHttpClient.Builder client = new OkHttpClient.Builder();        
        client.connectTimeout(60, TimeUnit.SECONDS);
        client.readTimeout(60, TimeUnit.SECONDS);
        client.writeTimeout(60, TimeUnit.SECONDS);

        mRetrofit = new Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(GsonConverterFactory.create())
                .client(client.build())
                .build();
    }

    public static MyNetworkService getInstance() {
        if (mInstance == null) {
            mInstance = new MyNetworkService();
        }
        return mInstance;
    }

    public MyJSONApi getJSONApi() {
        return mRetrofit.create(MyJSONApi.class);
    }
}

public interface MyJSONApi {
    @GET("idea/test")
    public Call<Idea> test();
}